If im right, the issue you have is that you have 3 cars. Unfortunately you only accrue no claims on 1 policy, so if you get another car then your back to square1 on NCB. I found this out when I got am old Golf GTI as my everyday car when I had the Elise. Despite havening no claims for ever, a clean record and the golf was worth <2k many companies would not even quote me (computer says no!) as they just work on the basis that no no-claims = huge risk
The way to do it is try specialist companies, especially anyone who does
competition/classic stuff and go for a limited miles policy possibly on all your "play cars"
I am currently with Haggarty Insurance (Elise and Series II)
http://www.hagertyinsurance.co.uk/contactus.aspxThey MAY not quote if they dont rate your Disco as old enough or classic enough, but they did quote me on adding a V8 90 to my poilcy for £50 as they work on the basis that you can only drive one car at time. I think they also need the car to be kept off the road overnight.
IF not try any of the specialists in the back of mags, but you need to talk to / find a propper broker, not just a call center operator as a broker may be able to explain the situation and get discount based on driving record etc.